The present invention relates to a process for the continuous, contact-free measurement of the thickness of a layer applied to a support and to an apparatus for performing the process.
The magazine "Adhaesion" [Adhesion] 1980, number 6, pages 183 to 185, describes systems for the continuous measurement of layer thicknesses, in particular of lacquers and adhesives applied in the liquid state. The measurement principles on which the various methods of thickness measurement are based, which have gained general acceptance in practical use, are compiled, for example, in DIN specifications No. 50,982, part 2, May 1978. Physical effects, which are suitable for a continuous, contact-free measurement of the thickness of material layers, are of optical or electrical nature or are based on the interaction of high-energy radiation, in general radioactive radiation, and material. If an electrical effect is used in apparatuses for the determination of layer thicknesses, the thickness measurement is based on a capacitance measurement. Thereby, the support, to which the layer is applied, must be electrically conductive, i.e., in general a metallic support is used, the support then being connected to the measurement system as one electrode. Together with a second electrode, i.e., the real measuring electrode, the support for the layer forms a plate capacitor system which, relaive to the distance between the two electrodes, has a large surface area. There exists also a contact-free variant of this process according to DIN specification No. 50,985, in which the actual measuring electrode is arranged as closely as possible above the surface of the layer to be measured. Thereby, the distance between the measuring electrode and the surface of the layer to be measured must be selected such that a contact between the layer surface and the measuring electrode is excluded, even if the thickness of the layer to be measured varies.
The capacitance C.sub.0 of the measuring capacitor can be determined from the distance "a" between the two electrodes and the surface area "F" of the measuring electrode, by means of the equation C.sub.0 =.epsilon..sub.0 .multidot.F/a with the layer to be measured being not yet inserted between the electrodes. The dielectric constant .epsilon..sub.r of the layer to be measured must be determined before by means of a suitable electrical measuring method. The layer thickness is then calculated using the formula ##EQU1## when the capacitance C formed by this plate capacitor arrangement is measured continuously, so that in this way the desired permanent thickness control of the layer conveyed on a metal web below the measuring electrode is achieved. Such an arrangement essentially serves to determine a mean layer thickness value, which is averaged over the surface area of the measuring electrode.
U.S. Pat. No. 4,451,732 describes a measuring system comprising a radiation source of radioactive isotopes and a Geiger-Muller Tube as a detection device, for measuring the coating thickness of a continuously moving strip material. The detection device measures, for example, the .beta.-radiation reflected from the strip material, the intensity of which depends on the layer thickness of the strip material.
In German Offenlegungsschrift No. 32 27 025, there is disclosed a device for determining the weight per unit area or the thickness of a coating applied to a moving strip material, in which a portion of the circumferential surface of a roller dips into the coating agent and transfers some of the coating agent onto the strip material or a distributor roller. Across the width of the strip material, the coating agent container is subdivided into several individual container sections, and the amount of coating agent consumed in each container section is measured separately. The measured values, corresponding to the coating agent consumption measured per time unit, are fed into a computer. The computer controls an automatic control system which adjusts the thickness or the weight per unit area, respectively, of the layer to a constant value by supplying the required quantity of coating agent to each container section, independently of the other container sections.
A disadvantage of the known capacitive measuring method resides in the fact that it largely depends on the distance between the measuring electrode and the surface of the layer to be measured. On the one hand, this distance should be as small as possible, whereas on the other hand it must be great enough to avoid any contact between the layer and the measuring electrode in the case of significant variations in the layer thickness. Employing this measuring method, the thickness of very thin layers is inaccurately determined because the process is not sufficiently sensitive. Furthermore, it is impossible with this method to measure the layer thicknesses of materials which are not purely dielectric, but which additionally possess a certain conductivity in the dark and in the light.